In recent years, there has been a remarkable spread of mobile phones and PDAs (Personal Digital Assistant). It has become a common practice to incorporate a compact digital still camera unit and digital video unit in such devices. These devices are characterized by severe restrictions on the size and cost. Therefore, there has been generally used a small-sized image pickup device equipped with a smaller number of pixels than that in a normal digital still camera, and a unifocal optical system including about one to three plastic lenses. In the image pickup device mounted on a mobile information terminal, technology is advancing rapidly toward a further increase in pixel level and functional level. In this context, there has been an intense demand for a compact variable-power optical system compatible with an image pickup element of high pixel level and capable of capturing the image of a subject located far away from a user, wherein this variable-power optical system can be mounted on a mobile phone and others.
The Patent Literature 1 discloses a three-component variable-power optical system with negative-positive-positive components wherein movement of a lens group resulting from varying power is minimized by increasing the varying-power load of the second lens group. The disclosed optical system, however, uses three lenses for each of the first and second lens groups, and a compact configuration is not sufficiently achieved in the optical system.
The Patent Literature 2 discloses a three-component variable-power optical system with negative-positive-negative components wherein compact configuration is realized by statically positions the first lens group. In the proposed optical system, however, the power arrangement in the first lens group and lens material of the second lens group are not optimized. Correction of the astigmatism and chromatic aberration is insufficient.
The Patent Literature 3 proposes a four-component variable-power optical system with negative-positive-negative-positive components, a so-called light bending optical system wherein the optical axis is bent perpendicularly and the optical unit is designed to have a slim configuration. However, this system contains as many as seven to nine lenses including the prism for bending the optical axis, and this results in increased costs. Although an attempt for thin configuration has been made, the overall length of the optical system is excessive, and efforts toward compact configuration are insufficient in terms of unit volume.    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2001-318311    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2005-55725    Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2004-205796